This invention relates to semiconductor devices with photoconductivity and more particularly to a semiconductor device having high photoconductivity for visible rays.
Selenium (Se) and cadmium sulphide (CdS) have hitherto been known as materials having high photoconductivity and so high a dark resistance that they can be applied with high electric field. However, these materials are poisonous and classified into a material source of environmental pollution and the advent of a semiconductor device using a substitutive, unpolluted material has long been desired.
Recently, amorphous silicon has been highlighted as a material which is unpolluted and yet has high photoconductivity. This material is, however, disadvantageous in that the resistivity at room temperature is of the order of 10.sup.13 .PSI..cm at the most, that a breakdown voltage in current-voltage characteristics is so low that a high voltage or a high electric field cannot be applied, and that the dark current density is large.
Also, Phil. Mag. page 7, Vol 35, 1978, suggests availability of amorphous silicon carbide, a-Si.sub.1-x C.sub.x, as an unpolluted material and discloses a preparation process thereof. It further teaches that as the concentration of carbon to be mixed with amorphous silicon is increased, the energy gap is increased before the carbon concentration reaches a certain limit so that the resistivity at room temperature can be increased. However, photoconductivity of this material has not yet been discussed in any papers.
Conventionally, it has been known to independently prepare either amorphous silicon film or amorphous silicon carbide film by sputtering or through plasma glow process. In one method, for example, a target silicon substrate is sputtered to deposit silicon on the other substrate, thereby forming an amorphous silicon film. In the other method, a silicon substrate carrying a lamp of carbon is sputtered to deposite silicon carbide by the reaction of sputtered carbon and silicon on the other substrate. However, to fabricate a device having a heterojunction between silicon and silicon carbide, the above known method requires an additional preparation step and it also has difficulties with varying the amount of each component in silicon carbide.